1. Field of the Invention
This invention relates to a retrofocus type wide angle objective lens in which aberrations are prevented from deteriorating even in short distance photography.
2. Description of the Prior Art
In general, the aberrational state of an optical lens varies with object distance. Ordinary photographic lenses are corrected for aberrations at infinity so that the closer the object position is with the photographing lens, the larger the aberrations become with the result that the picture quality on the film deteriorates.
This phenomenon is especially remarkable in optical systems of longitudinal asymmetry with respect to the diaphragm thereof and it has been a very serious problem in retrofocus type lenses as the field angle is large.
In general, the retrofocus type lens when focused to a close object produces spherical aberration, coma, astigmatism and curvature of field to a remarkable extent. Therefore, various techniques of correcting aberrations at short distances have been proposed.
However, most of these proposals are subjected to a limitation such that air spaces in which axial rays are parallel or substantially parallel with the optical axis be varied. This limitation is laid down for the purpose of producing to bad influence on the spherical aberration and back focus.
Although this method is effective for correction of astigmatism and curvature of field, generally speaking, it is difficult to effect good correction of coma. This is because the space defined as having parallel rays is not always coincidence with a space suitable for use in correction of coma, and because the degree of freedom is not sufficient so that it is difficult to obtain the space with parallel rays usuable for aberration correction. Even in this method, it is usual that the performance of the lens for close photography is not always satisfactory as compared with the performance for infinite photography.